The present invention generally relates to rotary machines, such as steam and gas turbines, and, more particularly, is concerned with a seal arrangement for a rotary machine such as a turbine.
Rotary machines, such as steam and gas turbines, used for power generation and mechanical drive applications are generally large machines having multiple turbine stages. In turbines, high pressure fluid flowing through the turbine stages must pass through a series of stationary and rotary components, and seals typically are used between the stationary and rotating components to minimize leakage of the fluid. The efficiency of the turbine is dependent on the ability of the seals to prevent fluid leakage.
Traditionally, one type of seal that is used to control fluid leakage in turbines is a labyrinth seal which has sharp rigid teeth. While the labyrinth seal has proven to be quite reliable, its performance degrades over time. This occurs as a result of transient events in which the stationary and rotating components move radially relative to and thus interfere with one another causing rubbing on the teeth of the labyrinth seal so as to enlarge the clearance between the stationary and rotating components and allow increase of leakage.
Another type of seal that is used in combination with the labyrinth seal to improve leakage control is a brush seal. The brush seal is generally less prone to leakage than the labyrinth seal because the brush seal has seal bristles that can flex and thus better accommodate the relative radial movement between the stationary and rotating components. The brush seal also generally conforms better to surface non-uniformities than does the labyrinth seal. Such combination brush and labyrinth seal arrangements are disclosed in U.S. Pat. No. 5,749,584 to Skinner et al. assigned to General Electric Company, the assignee of the present invention, and U.K. Pat. Application No. 2,301,635 to Hemsley et al assigned to GEC Alsthom Limited.
In one prior art high pressure differential brush seal design shown in FIG. 1, the brush seal 10 includes a layer of bristles 12 disposed or sandwiched between an upstream plate 14 and a downstream plate 16. The downstream plate 16 is a backing plate provided to prevent deflection of the bristles 12 under the loading from an upstream direction of the flow while an inner portion 12a of the layer of bristles 12 project beyond the inner edge 16a of the downstream plate 16 so as to engage the rotor R of the turbine. In order to achieve higher pressure differentials, a pressure balance design is required to relieve the net force holding the layer of bristles 12 against the downstream plate 16. The one prior art design also includes a seal holder 18 capturing the brush seal 10 and incorporating passageways 20 for directing high pressure steam to a downstream side of the layer of bristles 12 in order to pressurize a cavity 22 present between the layer of bristles 12 and the downstream plate 16.
A problem, however, exists with such a high pressure brush seal design. The brush seal 10 is bulky in size and thus difficult to retrofit into existing steam turbines where space is limited. Thus, a need exists for an innovation which will provide an effective solution to the aforementioned problem without introducing any new problems in place thereof
The present invention provides a seal arrangement for a rotary machine, such as a turbine, which incorporates a pressure balanced brush seal in a manner designed to satisfy the aforementioned need. In the present invention, the pressure balance element is a radial passage formed through the main seal segment (such as a labyrinth seal segment) which eliminates the need for the brush seal holder of the prior art design. As a result, the overall size of the brush seal is reduced and a more compact brush seal is achieved, permitting the installation of the brush seal in applications where space is limited.
In one embodiment of the present invention, a seal arrangement for a turbine is provided having a main seal segment and a brush seal segment. The main seal segment has an outer periphery, an inner periphery disposed radially inwardly from the outer periphery and a slot defined in the main seal segment between the outer and inner peripheries thereof and being open at the inner periphery and extending toward and terminating in a spaced relationship from the outer periphery. The brush seal segment is disposed in the slot and includes a layer of bristles having opposite outer and inner portions and upstream and downstream sides, an upstream plate having outer and inner portions and being disposed at the upstream side of the layer of bristles and a downstream plate having outer and inner portions and being disposed at the downstream side of the layer of bristles. The outer portion of the layer of bristles is secured between the outer portions of the upstream and downstream plates and the inner portion of the layer of bristles extends beyond the inner portions of the upstream and downstream plates. The downstream plate also has opposite ends, opposite sides respectively facing toward and away from the downstream side of the layer of bristles, and a cavity recessed in the one side thereof facing toward the downstream side of the layer of bristles. The cavity extends between and is terminated by the opposite ends of the downstream plate.
The seal arrangement further includes means for communicating pressure through the main seal segment to the cavity in the downstream plate of the brush seal segment from a source of pressure located exteriorly of the outer periphery of the main seal segment for providing a balance of pressure between the pressure source and the cavity of the downstream plate of the brush seal segment and thereby at the downstream side of the layer of bristles. The means for communicating pressure includes at least one elongated passage formed radially through the main seal segment between the outer periphery and the slot thereof and at least one hole formed through the downstream plate of the brush seal segment between the opposite sides of the downstream plate and communicating with the passage and opening into the cavity in the downstream plate. The hole is formed closer to the outer portion than to the inner portion of the downstream plate.
The main seal segment is arcuate-shaped for placing it end-to-end with other like main seal segments for extending about a turbine rotor. The brush seal segment is arcuate-shaped for placing it end-to-end with other like brush seal segments so as to conform to the arcuate-shaped main seal segment.